Corrugated plastic pipe is typically manufactured using an extrusion process involving a moving mold tunnel that determines the exterior shape of the pipe. The mold blocks are designed to apply a vacuum force to the extruded plastic to draw the plastic into contact with the interior shape of the mold blocks. These mold blocks are also designed to remove heat from the extruded plastic to generally set the corrugated pipe at the exit of the mold block tunnel.
In the case of corrugated extruded pipe having a double walled thickness, a first extruded plastic stream is acted upon by the moving mold tunnel to define the exterior profile and a second plastic stream is extruded to form an inner wall of the corrugated pipe. In the manufacture of double wall corrugated pipe, an interior cooling plug may also be used to assist in cooling the plastic by removing heat from the interior of the pipe.
Single walled corrugated plastic pipe and double walled corrugated plastic pipe are often used in drainage application and these pipes are typically buried and subject to substantial compressive forces. Conventional single or double walled corrugated plastic pipes are designed to withstand these compressive forces, and are commonly made of a circular cross section.
Corrugated plastic pipes have also been used for other applications such as conduit applications for wiring. These conduits may be imbedded in a concrete floor, for example, and are subject to less compressive forces. For conduit applications, the pipe may be made of a flexible material to allow for flexing or changes in direction of the conduit. Conduits may also be made of a more rigid material to improve the structural characteristics thereof. Conduits are also used as protective sheaths for wiring in non buried applications.
Double walled corrugated pipe is also practical for use as a ventilation duct. The outer corrugations of the pipe provide improvements in the structural characteristics of the product, and the smooth inner wall of the pipe provides an effective channel for fluid flow.
In the manufacture of corrugated plastic pipe, whether it is single walled or double walled, the ability of the moving mold tunnel to remove heat from the extruded plastic and set the plastic is critical to the quality control. The product at the exit of the moving mold tunnel must be at least partially set to reduce undesired deformation. The product at this time is subject to continued shrinkage as the plastic cools and the product is also subject to gravitational forces that can cause distortion.
In the manufacture of non-circular cross section corrugated pipe, and in particular in the manufacture of corrugated plastic pipe having a relatively straight wall segment, there is a tendency for this straight wall segment to distort inwardly. This inward distortion is a function of the gravitational force on the straight segment during the cooling of the product, as well as the tendency for the plastic to shrink. In extruded corrugated plastic pipe formed with a significant straight wall section it is difficult to avoid this distortion while maintaining a satisfactory speed of manufacture. Inward distortion also reduces the performance of the product in use.
The present invention seeks to overcome a number of difficulties associated with the manufacture of corrugated plastic pipe of either single or double walled configuration in which the cross section includes a substantial straight section.